The present invention relates to a microminiature moving device that is fabricated by micromachining technologies including photolithography, etching, etc. and, more particularly, to a microminiature moving device provided with moving parts that are freely displaceable in parallel to the substrate surface.
Microminiature moving devices of this kind include such, for example, as a microminiature optical switch, a microminiature accelerometer, and microminiature relay device. These microminiature moving devices are usually fabricated using a three-layered SOI (Silicon On Insulator) substrate that has an insulating layer sandwiched between a single-crystal silicon layer and a singe-crystal silicon substrate.
FIG. 1 shows, as a prior art example of the microminiature moving device, the optical switch described in U.S. Pat. No. 6,315,462 (issued Nov. 13, 2001, hereinafter referred to as document 1). FIG. 2 shows a sequence of steps involved in the manufacture of the optical switch disclosed in document 1. Referring first to FIG. 1, the configuration of the optical switch will be described.
In the top surface of a plate-like flat substrate 11, which is an SOI substrate, there are formed crosswise four optical fiber insertion channels 12a to 12d, providing an actuator forming area 11′ defined by the fiber insertion channels 12a and 12b extending at right angles to each other. In the actuator forming area 11′ there is formed a slot 13 extending at an angle of 45° with respect to the fiber insertion channels 12a and 12b, respectively. Disposed in the slot 13 is a movable rod 14.
The movable rod 14 carries at one end thereof a mirror 15, which is initially positioned at the intersection area 16 of the four fiber insertion channels 12a to 12d extending at right angles to one another. The movable rod 14 has coupled to its both sides intermediately thereof support beams 17a and 17b at one end thereof, the other ends of the support beams 17a and 17b being fixed to stationary parts 19a and 19b through leaf-spring-like support elements 18a and 18b, respectively. Similarly, support beams 17c and 17d are each coupled at one end to either side of the other end of the movable rod 14 opposite to its inner end, and the other ends of the support beams 17c and 17d are also fixed to the stationary parts 19a and 19b through leaf-spring-like support elements 18c and 18d, respectively. Thus the leaf -spring-like support elements 18a through 18d allow the movable rod 14 to move lengthwise thereof.
The movable rod 14 is driven by a comb-type electrostatic actuator, which is composed of movable comb electrodes 21a to 21d fixed to the support beams 17a to 17d, respectively, and stationary comb electrodes 22a to 22d fixedly formed in the actuator forming area 11′ so that they are interleaved with the movable comb electrodes 21a to 21d, respectively.
A voltage application between the movable comb electrodes 21a, 21b and the stationary comb electrodes 22a, 22b creates an electrostatic attractive force, driving the movable rode 14 toward the afore-mentioned intersection area 16. On the other hand, a voltage application between the movable comb electrodes 21c, 21d and the stationary comb electrodes 22c, 22d creates an electrostatic attractive force, driving the movable rod 14 away from the intersection area 16. Accordingly, driving the movable rod 14 by the comb-type electrostatic actuator through selective voltage application allows the mirror 15 to be pushed into and pulled out of the intersection area 16.
In the four fiber insertion channels 12a to 12d there are disposed optical fibers 23a to 23d, respectively. With the mirror 15 held at the intersection area 16, light emitted from the optical fiber 23a, for instance, reflects off the mirror 15 and impinges on the optical fiber 23d, and light emitted from the optical fiber 23b also reflects off the mirror 15 and strikes on the optical fiber 23c. With the mirror 15 pulled out of the intersection area 16, the light emitted from the optical fiber 23a is incident on the optical fiber 23c, and the light emitted from the optical fiber 23b is also incident on the optical fiber 23d. In this way, the optical path is switched.
FIG. 2 shows a sequence of steps S1 to S3 involved in the manufacture of the conventional optical switch described above. The manufacture starts with step S1 in which an SOI substrate 30 is prepared which has a silicon dioxide (SiO2) insulating layer 32 sandwiched between a single-crystal silicon layer 33 and a single-crystal silicon substrate 31, which is followed by the formation of a mask 34 of a required pattern over the entire surface area of the single-crystal silicon layer 33 by patterning. Then in step S2 the single-crystal silicon layer 33 is selectively etched away down to the surface of the insulating layer 32 by reactive ion etching (RIE) through the mask 34, leaving the required pattern all over the insulating layer 32.
The thin webs 35 of the single-crystal silicon layer 33 formed in step S2 correspond to such movable elements as the movable rod 14, the mirror 15, the support beams 17a to 17d, the leaf-spring-like support elements 18a to 18d and the movable comb electrodes 21a to 21d in FIG. 1, whereas the thick webs 36 will ultimately form the stationary parts 19a, 19b and other stationary members that are fixed to the single-crystal silicon substrate 31 in FIG. 1. In FIG. 2 the movable elements and the stationary parts are merely exemplified.
Then in step S2 the exposed insulating layer 32 is subjected to wet etching until the insulating layer 32 underlying the thin webs 35 is removed by side etching. As a result, the thin webs 35 are suspended slightly above the single-crystal silicon substrate 31 as indicated by 37 in step S3; that is, the movable elements formed by the thin webs 35 are severed by the selective removal of the insulating layer 32 from the single-crystal silicon substrate 31 so that they are freely displaceable. The thin web 35 of the single-crystal silicon layer 33, which forms the mirror 15, is coated on both sides thereof with reflecting films.
With the manufacturing method shown in FIG. 2, the insulating layer 32 of the SOI substrate 30, which underlies the movable elements formed by the single-crystal silicon layer 33, is etched away to permit displacement of the movable elements.
As described above, the microminiature moving device of the type having movable elements displaceable in parallel to the substrate surface is usually fabricated using the SOI substrate 30, and the intermediate insulating layer 32 is selectively etched away to form the movable elements (thin webs 35) each separated by a slight gap from the underlying single-crystal silicon substrate 31.
Since the intermediate insulating layer 32 of the SOI substrate 30 is typically 3 μm or so at a maximum, however, the gap between the movable elements (thin webs 35) and the single-crystal silicon substrate 31 is so narrow that even the smallest foreign body in the gap will be likely to disturb normal working of the movable elements.
Furthermore, at the same time as the insulating layer 32 underlying the movable elements (thin webs 35) is etched away, the insulating layer 32 underlying the stationary parts (thick webs 36) is also removed by side etching to form between the stationary parts (thick webs 36) and the underlying single-crystal silicon substrate 31 a gap of the same width as that of the side etching of the insulating layer 32 underlying the movable parts (thin webs 35); hence, foreign bodies may sometimes get into the gap, too. In this instance, there is a fear of shorting the single-crystal silicon substrate 31 and the stationary parts (thick webs 36) via the foreign bodies—this may occasionally produce the situation of shorting the stationary parts (thick webs 36) via the foreign bodies and the single-crystal silicon substrate 31.
Accordingly, the conventional microminiature moving device has the defect of accidental shorting and hence malfunction of the two stationary parts that are used as electrodes for voltage application to the comb-type electrostatic actuator.